A rapidly spreading wildfire three times the size of San Francisco roared in to Yosemite National Park on Friday, fanning fears that California would actually see the fiery summer many envisioned following the bone-dry winter.

On Thursday, the Rim Fire nearly quadrupled in size in 24 hours to more than 63,000 acres, or one-fifth the total land burned in California so far this year, prompting tourists to flee the Yosemite area, closing the main highway access to the renowned national park and damaging the Hetch Hetchy water and power system. The Strawberry Music Festival at Camp Mather, a popular bluegrass destination for Bay Area music fans scheduled for next week, was canceled.

Overnight into Friday morning, the Rim Fire nearly doubled in size again to about 106,000 acres, or almost the size of San Jose. The number of homes threatened jumped from 2,500 to 4,500 Friday and the fire crossed over the Yosemite National Park boundary for the first time.

Gov. Jerry Brown declared a state of emergency for Tuolumne County, five days after the fire started, saying that statewide resources were needed to help with evacuations and emergency shelters as the cost to fight the fire hit $5.4 million. Sixteen structures were destroyed, and one firefighter suffered heat exhaustion. The cause of the blaze is under investigation.

Bay Area crews were among the 2,000 firefighters called in to douse the blaze, which workers had only 2 percent contained as they struggled to get fire trucks into the steep canyons bordering Yosemite.

"It's extremely difficult terrain to work in," said Dennis Mathisen, a spokesman for Cal Fire, which is leading the joint effort of state, federal and local firefighters. "It just takes a lot of resources and time to get to where they need to be."

The blaze is even more significant because California had thus far gotten off pretty easy this fire season.

Across the Golden State, 216,948 acres of state and federal land had burned in wildfires this year through Tuesday, just before the Rim Fire exploded in size, according to the U.S. Forest Service. But even when including the Rim Fire, that's still fewer than half the 529,761 acres that had burned by this time last year -- and about half the amount of land charred on average over the last five years.

"In general, drier years are worse fire years, and this is a drier year. But you need lots of other things" such as high temperatures, low humidity and winds, said Jan Null, a forecaster with Golden Gate Weather Services in Saratoga. Plus, when you factor in human mistakes and arson, "there's a lot of randomness that goes into it beside the weather."

The fire season was expected to be worse after Californians enjoyed sunny skies much of the winter. San Francisco, at 3½ inches of rain since January, had the driest start to the year on record since 1850, Null said. San Jose's rainfall during the past year was 60 percent of normal and Oakland's was 70 percent of normal, mirroring trends around the state, according to the National Weather Service.

But the Bay Area and other parts of California, including the Sacramento and Fresno areas, were a couple of degrees cooler on average in August, Null said, helping keep the risk of fires low.

Now, as the Diablo and Santa Ana winds start to whip up, the fire danger could change quickly, said Craig Clements, director of the Fire Weather Research Lab at San Jose State University. September and October in California are typically the worst two month for fires as timber reaches its driest state.

"We are getting into more critical conditions," Clements said. "The total acres burned this year? That could triple by Sept. 1 or Sept. 15."

Fire officials fear that the busy season kicked off with the Rim Fire, which spread through the tiny rural communities around Groveland. Yosemite, though, was mostly untouched.

Still, a main route to the park from the Bay Area, Highway 120, was closed for four miles, forcing detours that officials estimated would tack on 15 to 20 minutes per drive. The park also closed its Hodgdon Meadow Campground, which includes 105 campsites, and was monitoring the fire as it stood about five miles from Yosemite on Thursday afternoon.

"Everything basically is business as usual," Yosemite spokesman Scott Gediman said. While smoke is visible from the western edge of the park, he said, "Where I am, in Yosemite Valley, it's crystal clear blue skies."

The San Francisco Public Utilities Commission, which owns the Hetch Hetchy system that originates at a reservoir in Yosemite, said despite damage to its infrastructure, water quality and supply was not affected Thursday.

The city was buying extra power to make up for hydropower plants that went offline and typically power municipal buildings, schools and San Francisco International Airport. And officials were monitoring ash that threatened to float into the reservoir for the system that supplies water to 2.6 million Bay Area residents and businesses.

Tuolumne County residents and businesses braced for the worst.

"This fire, it's killing our financial picture," said Corinna Loh, whose family owns the still-open Iron Door Saloon and Grill in Groveland. "This is our high season and it has gone to nothing; we're really hurting."

Loh said most of her employees have left town. And the family's Spinning Wheel Ranch, where they rent cabins to tourists, has also been evacuated because it's directly in the line of fire. Two outbuildings have burned at the ranch, Loh said, and she still has no word whether the house and cabins survived.

"We're all just standing on eggshells, waiting," Loh said.

The Associated Press contributed to this report. Contact Mike Rosenberg at 408-920-5705. Follow him at twitter.com/RosenbergMerc.